Geologic time is measured through the radioactive decay process, determining
the radiometric age of rocks in years. A geomagnetic time scale has
been established from the number and duration of reversals in the magnetic
field of Earth's past.
Correlation and the determination of the numerical ages of rocks
and events have led to the development of a geologic time scale. The major
blocks of time on this calendar are called eons. The Hadean eon, 3.8
to 4.5 billion years ago, is the period of time before Earth's surface solidified.
The Archean eon, 2.5 to 3.8 billion years ago, is a time when the atmosphere
lacked oxygen and only certain kinds of prokaryotic organisms
existed. The Proterozoic eon, 2.5 billion to 540 million years ago, saw the
development of an oxidizing atmosphere, the first eukaryotic cells, and
the first multicellular organisms. The time encompassed by the Hadean,
Archaean, and Proterozoic is also referred to as the Precambrian. The
Phanerozoic eon, 540 million years ago to the present, is a period of time
that has seen the elaboration of the various kinds of living things we see
today as well as many groups that have gone extinct. The Phanerozoic eon
is divided into smaller units of time known as eras. The eras are the
(1) Cenozoic, the time of recent life; (2) Mesozoic, the time of middle life;
and (3) Paleozoic, the time of ancient life. The eras are divided into
smaller blocks of time called periods, and the periods are further subdivided
into epochs.
There are many different kinds of fossils—evidence of former living
things. They are formed when organisms are covered over and prevented
from being destroyed.Most are modified chemically but still give information
about past living things. The fossil record is seen to change over
geologic time with certain kinds of fossils being associated with certain
periods of time. There have been several great extinctions of living things.
Some of the extinctions eliminated more than 80 percent of the organisms
alive at the time. These extinctions appear to be related to changes
in climate that may have been initiated by changes in the location and
arrangement of continents, volcanic activity, or meteorite impact.
The early evolution of humans has been difficult to piece together
because of the fragmentary evidence. Beginning about 4.4 million years
ago, the earliest forms of Australopithecus and Paranthropus showed upright
posture and other humanlike characteristics. The structure of the
jaw and teeth indicates that the various kinds of australopiths were herbivores.
Homo habilis had a larger brain and appears to have been a scavenger.
Several other species of the genus Homo arose in Africa. These
forms appear to have been carnivores. Some of these migrated to Europe
and Asia. The origin of Homo sapiens is in dispute. It may have arisen in
Africa and migrated throughout the world or evolved from earlier ancestors
found throughout Africa, Asia, and Europe.
To facilitate accurate communication, biologists assign a specific
name to each species that is cataloged. The various species are cataloged
into larger groups on the basis of similar traits. Taxonomy is the science
of classifying and naming organisms. Phylogeny is the science of trying to
figure out the evolutionary history of a particular organism. The taxonomic
ranking of organisms reflects their evolutionary relationships. Fossil
evidence, comparative anatomy, developmental stages, and biochemical evidence
are employed in the sciences of taxonomy and phylogeny.
The first organisms thought to have evolved were single-celled,
prokaryotic organisms. These simple organisms gave rise to two different
prokaryotic domains, Eubacteria and Archaea. The Eubacteria are a very
diverse group of organisms. Some are autotrophs, while others are heterotrophs.
The cyanobacteria were probably the first organisms to carry on photosynthesis. Many kinds of heterotrophic Eubacteria are decomposers, while some are parasites, mutualistic, or commensal organisms. The Archaea are distinguished by their ability to live in very hostile environments. Eukaryotic organisms were probably derived from the Eubacteria
and Archaea and constitute the domain Eucarya. There are four
kingdoms in the Eucarya: Protista, Fungi, Plantae, and Animalia. The
Protista includes organisms such as protozoa and algae that consist of
single cells or small colonies of cells. The Fungi kingdom includes multicellular,
heterotrophic organisms with cell walls made of chitin. The
Plantae are complex, multicellular, autotrophic organisms that have cell
walls made of cellulose and are primarily terrestrial. The Animalia are
complex, multicellular, heterotrophic organisms that lack cell walls and
typically have the ability to move.
